TOKYO-Japan's Princess Mako has formally announced her engagement to a non-royal after receiving the emperor's approval.

This announcement kicks off a lengthy marriage process, and it also means the princess will lose her royal status.

Under a controversial Japanese law, female imperial family members forfeit their status upon marriage to a "commoner" whereas male members do not.

At a press conference, she said she was first attracted to Kei Komuro's "smile like the sun".

"I've been aware since my childhood that I would lose royal status once I married," Princess Mako said. "While I've worked to help the emperor and fulfil duties as a royal family member as much as I can, I've been cherishing my own life."

The formal announcement came from Japan's Imperial Household Agency on Sunday, after local media reported news of the planned engagement in May.

The princess was not in line to the throne, as a 1947 succession law states that only men from her family's lineage can become emperor.

Her fiancé, Kei Komuro, is a 25-year-old law firm employee.

The couple met five years ago while studying at the same university.

In the televised conference, he described the princess as someone who quietly watched over him "like the moon".

Princess Mako, 25, is the eldest child of Prince Fumihito, whose official title is Prince Akishino. She is pursuing a doctorate and works as a museum researcher.

The announcement was originally expected in July, but was postponed after a rain disaster hit western Japan. The wedding is expected to take place next year, according to the public broadcaster NHK.